Tungsten-copper composite powder

ABSTRACT

A high performance W--Cu composite powder which is composed of individual particles having a tungsten phase and a copper phase wherein the tungsten phase substantially encapsulates the copper phase. The tungsten-coated copper composite powder may be pressed and sintered into W--Cu pseudoalloy articles having a homogeneous distribution of W and Cu phases without experiencing copper bleedout or it may be used in ceramic metallization for the electronics industry.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 08/559,903, filedon Nov. 17, 1995, which is a continuation-in-part of application Ser.No. 08/362,024 now U.S. Pat. No. 5,468,457 and Ser. No. 08/361,415 nowU.S. Pat. No. 5,470,549, both filed Dec. 22, 1994, the disclosures ofwhich are incorporated herein by reference.

BACKGROUND ART

Tungsten-copper (W--Cu) pseudoalloys are used in the manufacture ofelectrical contact materials and electrodes, thermal management devicessuch as heat sinks and spreaders, and conductive inks and pastes forceramic metallization. The basic methods for the fabrication of articlescomposed of W--Cu pseudoalloys include: infiltration of a poroustungsten skeleton with liquid copper, hot pressing of blends of tungstenand copper powders, and various techniques incorporating liquid phasesintering, repressing, explosive pressing, and the like. Complex shapesmay be made by injection molding W--Cu composite powders. It isdesirable to be able to manufacture articles made from W--Cupseudoalloys at or near the theoretical density of the pseudoalloy.Besides having improved mechanical properties, the higher densitypseudoalloys have higher thermal conductivities which are critical forthe application of W--Cu pseudoalloys as heat sink materials for theelectronics industry.

The components in the W--Cu system exhibit only a very smallintersolubility. Thus, the integral densification of W--Cu pseudoalloysoccurs above 1083° C. in the presence of liquid copper. The compressivecapillary pressure generated by the forming and spreading of liquidcopper, the lubrication of tungsten particles by liquid copper and theminute solubility of tungsten in copper above 1200° C. combine to causethe relative movement of tungsten particles during sintering and therebymake possible the displacement of tungsten particles. Localdensification and rearrangement of the tungsten framework causes aninhomogenous distribution of W and Cu phases in the sintered article andcopper bleedout, i.e. the loss of copper from the sintered article. Thisleads to the degradation of the thermal/mechanical properties of thesintered article.

Prior art methods directed to improving the homogeneity of W--Cucomposite powders by coating tungsten particles with copper have notbeen successful as these copper-coated powders still exhibit a hightendency towards copper bleedout during the consolidation of thecomposite powder into fabricated shapes.

Thus, it would be advantageous to eliminate copper bleedout fromoccurring during the liquid-phase sintering of W--Cu pseudoalloys whileproviding a homogeneous distribution of W and Cu phases in the sinteredarticle.

SUMMARY OF THE INVENTION

It is an object of the invention to obviate the disadvantages of theprior art.

It is another object of the invention to produce a W--Cu compositepowder which can be used to make W--Cu pseudoalloys having highelectrical and thermal conductivities.

It is a further object of the invention to produce a W--Cu compositepowder which may be pressed and sintered to near theoretical densitywithout copper bleedout.

It is still a further object of the invention to produce a W--Cucomposite powder which may be used to make sintered articles having ahigh degree of dimensional control.

In accordance with one object the of invention, there is provided atungsten-copper composite powder comprising individual particles havinga tungsten phase and a copper phase wherein the tungsten phasesubstantially encapsulates the copper phase.

In accordance with another object of the invention, there is provided aW--Cu composite oxide powder comprising individual particles having acopper tungstate phase and tungsten trioxide phase wherein the tungstentrioxide phase exists primarily at the surface of the individualparticles.

In accordance with a further object of the invention, there is provideda method for forming a homogeneous W--Cu pseudoalloy comprising pressinga tungsten-coated copper composite powder to form a compact andsintering the compact.

In accordance with a still further object of the invention, there isprovided a W--Cu pseudoalloy having a microstructural cross-sectionhaving tungsten areas and copper areas, the tungsten areas being lessthan about 5 μm in size and the copper areas being less than about 10 μmin size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photomicrograph of a cross-section of a tungsten-coatedcopper composite powder containing 25 weight percent copper.

FIG. 2 is a photomicrograph of a cross-section of a tungsten-coatedcopper composite powder containing 15 weight percent copper.

FIG. 3 is an illustrative view of a cross-section of a tungsten-coatedcopper composite particle wherein the tungsten phase substantiallyencapsulates the copper phase.

FIG. 4 is an illustrative view of a cross-section of a tungsten-coatedcopper composite particle wherein the tungsten phase completelyencapsulates the copper phase.

FIG. 5 is an illustrative view of a cross-section of a tungsten-coatedcopper composite particle having a dendritic morphology wherein thetungsten phase substantially encapsulates the copper phase.

FIG. 6 is a graphical illustration of the relationship between the molarratio of tungsten trioxide to copper tungstate and the X-ray Diffractionpeak intensity ratio of tungsten trioxide (3.65 Å) to copper tungstate(2.96 Å) for synthesized W--Cu composite oxides and mechanical mixturesof copper tungstate and tungsten trioxide.

FIG. 7 is a graphical representation of the relationship between thecompacting pressure and the electrical conductivity at differentsintering temperatures for a W--Cu pseudoalloy made from atungsten-coated copper composite powder having 15 weight percent copper.

FIG. 8 is a graphical representation of the relationship between thecompacting pressure and the electrical conductivity of W--Cupseudoalloys made from tungsten-coated copper composite powderscontaining varying amounts of copper.

FIG. 9 is a photomicrograph of a cross-section of a W--Cu pseudoalloyhaving 15 wt. % Cu which was made from a milled and spray driedtungsten-coated copper composite powder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims taken inconjunction with the above-described drawings.

Several factors influence the solid-state (below 1083° C.--the meltingpoint of copper) and liquid-phase (above the melting point of copper)sintering behavior of submicron W--Cu powder systems. Compactedrefractory metal powders undergo considerable microstructural changesand shrinkage during solid-state sintering (in the absence of liquidphase). Submicron particle size powders effectively recrystallize andsinter at temperatures (T) which are much lower than the meltingtemperatures (T_(m)) of refractory metals (T≅0.3 T_(m)) The initialsintering temperature for submicron (0.09-0.16 μm) tungsten powder is inthe range of 900-1000° C. The spreading of copper and the formation of amonolayer copper coating on tungsten particles occurs in the temperaturerange of 1000-1083° C. By lowering the activation energy for tungstendiffusion, monolayer copper coatings activate the solid-state sinteringof tungsten. Therefore, a number of complementary conditions are met forbonding submicron tungsten particles into a rigid tungsten frameworkwithin the composite powder compact during solid-state sintering(950-1080° C.). High fineness and homogeneity of the starting compositepowders are expected to enhance the sintering of a structurallyhomogeneous tungsten framework. Such framework should, in turn, aid inmaking a homogeneous pseudoalloy.

Such an idealized mechanism is complicated by local densification. Dueto nonuniform distribution of stresses at compaction of the startingpowder, submicron tungsten particles may experience rapid densificationin local regions. An unevenly sintered tungsten framework will causestructural inhomogeneity, localization of integral densification, andcopper bleedout at the liquid-phase sintering stage. Liquid-phasesintering comprises three stages: particle rearrangement, grain growthby a dissolution-reprecipitation mechanism, and the formation anddensification of a rigid skeleton. The operating mechanism for integraldensification in W--Cu systems consists of particle rearrangement withthe rate of densification inversely proportional to particle size andgrain shape accommodation for solid particles aided by minute solubilityof tungsten in copper at 1200° C. and above.

Integral densification of W--Cu pseudoalloys strongly depends oncapillary pressure generated by the liquid copper. This pressureincreases with the reduction of the liquid phase wetting angle. Thewettability of tungsten by copper improves with temperature. At 1100° C.the wetting angle is already substantially below 90°, and it steadilydiminishes with temperature to a value close to zero at 1350° C. Withimprovement of the wettability, the dihedral angles between adjacenttungsten particles attain values necessary for the liquid to penetratethe space between the particles and force them to slide relative to eachother. The rate and extent of integral densification and rearrangementof the tungsten framework are also affected by the amount of liquidphase and the size of tungsten particles.

We have discovered that a homogeneous sintered W--Cu pseudoalloy articlecan be formed without copper bleedout by using a W--Cu composite powdercomposed of tungsten-coated copper composite particles. Because theindividual particles have a tungsten outer layer and a copper core, W--Wcontacts dominate during compaction of the composite powder. Thisresults in forming a network of interconnected submicron tungstenparticles within the powder compact immediately after powderconsolidation. Solid-state sintering of the powder compact results in astructurally homogeneous tungsten framework which enables uniforminternal infiltration of the framework with copper during liquid-phasesintering and the formation of a dense pseudoalloy with a continuoustungsten/copper structure. As used herein, the term "tungsten-coated" or"W-coated" means that the tungsten phase substantially encapsulates thecopper phase. Unless otherwise indicated, all of the W--Cu compositepowders discussed below are composed of tungsten-coated copperparticles.

The preferred W--Cu composite powder has a Fisher Sub-Sieve Sizer (FSSS)particle size range of about 0.5 μm to about 2.0 μm in the coppercontent range of 2 to 25 wt. %. Each composite particle has a tungstencoating whose average thickness varies with the size and shape of theparticle and ranges between about 0.1 to about 0.2 μm.

FIGS. 1 and 2 are Back-scattered Electron Images (BEI) of cross-sectionsof tungsten-coated copper powders containing 25 and 15 wt. % copper,respectively. The BEI photomicrographs clearly show that the tungstenphase which appears as white has substantially encapsulated the copperphase which appears as gray. The black areas between the particles arefrom the epoxy mounting material. FIGS. 3, 4, and 5 are illustrations ofcross-sections of individual tungsten-coated copper composite particles.FIG. 3 shows the tungsten phase 10 substantially encapsulating thecopper phase 12. FIG. 4 shows a particle where the W phase 10 hascompletely encapsulated the Cu phase 12. FIG. 5 shows a particle havinga dendritic morphology wherein the W phase 10 has substantiallyencapsulated the Cu phase 12. At least 50% of the surface of the Cuphase is encapsulated by the W phase. Preferably, the tungsten-coatedcopper composite particles have at least 70% of the surface of thecopper phase encapsulated by the W phase. More preferably, at least 90%of the surface of the Cu phase is encapsulated by the W phase.

A preferred method for forming the tungsten-coated copper compositeparticles involves the hydrogen reduction of a W--Cu composite oxidepowder containing CuWO₄ and WO₃. Such a composite oxide may be made byreacting an ammonium tungstate, such as ammonium paratungstate (APT) orammonium metatungstate (AMT), with an oxide or hydroxide of copper.Methods for forming W--Cu composite oxides are described in copendingpatent application Ser. Nos. 08/362,024 and 08/361,415, both filed Dec.22, 1994, the disclosures of which are incorporated herein by reference.

In order to form an excess of WO₃ in the composite oxide, the amount ofammonium tungstate used is greater than the stoichiometric amount whichwould be required to form copper tungstate (CuWO₄) . Such a reactionforms an intimate mixture of CuWO₄ and WO₃ phases having a relativecopper content less than about 25 percent, i.e. relative to tungsten(CuWO₄ has a relative copper content of 25.7%). A homogeneous W--Cucomposite oxide powder having a composition of CuWO₄ +(0.035-15) WO₃ anda relative copper content of about 25% to about 2% is the preferredmaterial to make the tungsten-coated copper composite powder. Examplesof the synthesis of W--Cu composite oxides having variable coppercontent are given in Table 1.

                  TABLE 1    ______________________________________                         Relative Copper Content    Reaction             in W-Cu Composite Oxide    ______________________________________    0.547 AMT + 0.5 Cu.sub.2 O + 0.25 O.sub.2 →                         5.0%    CuWO.sub.4 + 5.566 WO.sub.3    0.259 AMT + 0.5 Cu.sub.2 O + 0.25 O.sub.2 →                         10.0%    CuWO.sub.4 + 2.11 WO.sub.3    0.163 AMT + 0.5 Cu.sub.2 O + 0.25 O.sub.2 →                         15.0%    CuWO.sub.4 + 0.958 WO.sub.3    0.115 AMT + 0.5 Cu.sub.2 O + 0.25 O.sub.2 →                         20.0%    CuWO.sub.4 + 0.382 WO.sub.3    0.083 AMT + 0.5 Cu.sub.2 O + 0.25 O.sub.2 →                         25.7%    CuWO.sub.4    ______________________________________

It is believed that the encapsulation of the copper phase by thetungsten phase starts at the composite oxide synthesis stage with theformation of a WO₃ -coated CuWO₄ core within every discrete particle ofthe W--Cu composite oxide powder. It has been observed that reactions ofammonium tungstates with oxides of copper produce composite oxidepowders having particle morphologies which mimic the morphology of theammonium tunsgtate particles. This is similar to the effect observed inthe production of tungsten powder by hydrogen reduction of WO₃. There,the morphology of the tungsten powder is controlled by the morphology ofthe WO₃ which, in turn, is strongly influenced by the morphology of theammonium tungstate powders from which the WO₃ is made.

I. W--Cu COMPOSITE OXIDE PRECURSOR

W--Cu composite oxide powders having a composition of CuWO₄ +0.958 WO₃were synthesized from (1) Cu₂ O+precipitated APT (angular particlemorphology) and (2) Cu₂ O+spray-dried AMT (spherical particlemorphology). Both APT and AMT powders were converted to composite oxideparticle agglomerates which were isomorphic with the starting particlesof APT and AMT. The particles of cuprous oxide totally disappearedduring synthesis of the composite oxide. It was not possible to identifyany effect of the starting morphology of Cu₂ O on the morphology of thecomposite oxide powder. The small, mostly submicron, grains forming eachparticle of the W--Cu composite oxide suggest that WO₃ grains outlinethe diffusion boundaries for the solid-phase reaction. It is believedthat the reaction mechanism consists of the diffusion of mobilemolecules of copper oxide into WO₃ grains. The ability to control theW--Cu composite oxide morphology by varying the morphology of ammoniumtungstates used in synthesis can be used to influence the kinetics ofthe hydrogen reduction of the oxides and the morphology of co-reducedW--Cu composite powders.

W--Cu composite oxides having a composition of CuWO₄ +nWO₃ (n=0.035-15)and a relative copper content in the range of 25 to 2% have beensynthesized from Cu₂ O and spray-dried AMT. A number of these compositeoxides and stoichiometric copper tungstate formed by the same method(25.7 to 5% Cu, n=0-5.566) were subjected to elemental analysis byEnergy Dispersive X-ray Spectroscopy (EDS) and to phase analysis byX-Ray Diffraction (XRD). The EDS analysis included measurement of W--Lαto Cu--Kα peak intensity ratios from which W/Cu atomic ratios for thedifferent oxide compositions were calculated for single particles perfield and groups of 2 to 5 particles of different sizes per field. Theanalytical results for populations of single particles are given inTable 2. A good correlation between experimental and theoretical W/Cuatomic ratios were found for all analyzed particle sizes of thestoichiometric CuWO₄ (n=0) composite oxide. However, for the single-sizeparticle populations corresponding to the CuWO₄ +nWO₃ (n=0.382-5.566)composite oxides, the experimental W/Cu atomic ratio deviatedsignificantly from the theoretical W/Cu atomic ratio. Table 2 shows thatthe size of the deviation grows with increasing WO₃ content and isespecially pronounced for 1 μm particles. Tungsten, copper, and oxygenpeaks were present in every analyzed particle which confirmed that theparticles formed in the course of solid-phase synthesis are W--Cucomposite oxides as opposed to mechanical mixtures of CuWO₄ and WO₃.

                                      TABLE 2    __________________________________________________________________________    Theoretical Data      Experimental Average W/Cu               Relative                     W/Cu Atomic Ratio for Populations    Oxide      Copper                     Atomic                          With a Particle Size of, μm    Composition               Content, %                     Ratio                          1   5   15 30    __________________________________________________________________________    CuWO.sub.4 25.7  1.000                          0.963                              0.957                                  0.956                                     0.949    CuWO.sub.4 + 0.382 WO.sub.3               20    1.382                          1.553                              1.476                                  1.977                                     1.502    CuWO.sub.4 + 0.958 WO.sub.3               15    1.958                          3.649                              2.629                                  3.351                                     3.042    CuWO.sub.4 + 2.11 WO.sub.3               10    3.110                          13.909                              7.203                                  7.677                                     3.672    CuWO.sub.4 + 5.566 WO.sub.3               5     6.566                          22.338                              9.779                                  8.538                                     10.738    __________________________________________________________________________

A better correlation between theoretical and experimental W/Cu atomicratios is found when 2 to 5 particles of different sizes are analyzed.Thus, there is an improvement in material homogeneity with an increasein the number of analyzed particles. The homogeneity of populations ofparticles can be expressed as a coefficient of variation which isdefined as the ratio of the population standard deviation to its meanexpressed in percent. Table 3 gives the coefficients of W/Cu atomicratio variation for populations of single 1 μm particles (Column A) andpopulations of 2 to 5 particles of different sizes (Column B).

                  TABLE 3    ______________________________________    W-Cu Composite Oxide    Composition         A      B    ______________________________________    CuWO.sub.4          3.0    4.9    CuWO.sub.4 + 0.382 WO.sub.3                        122.9  12.3    CuWO.sub.4 + 0.958 WO.sub.3                        141.4  43.2    CuWO.sub.4 + 2.11 WO.sub.3                        82.6   30.9    CuWO.sub.4 + 5.566 WO.sub.3                        29.9   17.0    ______________________________________

The deviation from CuWO₄ stoichiometry results in a reduction of theW--Cu composite oxide homogeneity for particle sizes of about 1 μm(Column A). Homogeneity for the single 1 μm particles improves withincreasing amounts of WO₃. The effect is less pronounced when largerparticles or larger number of particles are analyzed (Column B). Thevariations in homogeneity disappear entirely when thousands of particlesare examined simultaneously as with Sputtered Neutral Mass Spectrometry(SNMS) analysis.

XRD phase analysis of synthesized composites were compared to XRDstandards made of mechanical blends of CuWO₄ and WO₃ powders containingthe same proportions of the two phases. FIG. 6 shows that the WO₃ /CuWO₄peak intensity ratio is consistently greater for the synthesized CuWO₄+nWO₃ (n=0.382-5.566) composite oxides compared to the correspondingmechanical blends.

The EDS and XRD results for the CuWO₄ +nWO₃ (n=0.382-5.566) compositeoxides indicate that fine submicron CuWO₄ particles are being surroundedby WO₃ particles within 1 μm particle agglomerates. It is believed thatthis encapsulation of the CuWO₄ phase by the WO₃ phase results in theformation of finely dispersed tungsten-coated copper particles duringhydrogen reduction of the composite oxide.

Forming the W--Cu composite oxides from ammonium tungstates and oxidesor hydroxides of copper yields additional benefits. The high surfaceareas and additional reactivity generated from the thermal decompositionof these reactants improves the degree of mixing, shortens the diffusiondistances, and results in sufficient diffusion activities to promotesynthesis reactions in every discrete grain of WO₃ which is produced byin-situ thermal decomposition of the ammonium tungstate. This allows theuse of a broad range of particle sizes for the solid reactants whichmakes the process quite forgiving.

To illustrate this, CuWO₄ +0.958 WO₃ composite oxides (15% Cu) weresynthesized from mechanical blends of spray-dried AMT and Cu₂ O. Threefeedstocks (A, B, C) for hydrogen reduction were synthesized having anearly 2 to 1 variation in the median AMT particle size and a nearly 15to 1 variation in the median Cu₂ O particle size. Concomitant with thesize variation, there was a corresponding substantial variation of thesurface area of solid reactants. The surface areas and particle sizes(90th, 50th, and 10th percentiles as determined by Microtrac) are givenin Table 4.

                                      TABLE 4    __________________________________________________________________________    SOLID-PHASE SYNTHESIS OF W-Cu COMPOSITE OXIDES (15% Cu) AT 775°    C.    Tungsten Source  Copper Source                                  Composite Oxide    (AMT)            (Cu.sub.2 O) (CuWO.sub.4 + 0.958 WO.sub.3)                 Specific     Specific     Specific        Particle Size                 Surface                     Particle Size                              Surface                                  Particle Size                                           Surface    FEED-        Distribution, μm                 Area,                     Distribution, μm                              Area,                                  Distribution, μm                                           Area,    STOCK        D90           D50              D10                 m.sup.2 /g                     D90                        D50                           D10                              m.sup.2 /g                                  D90                                     D50                                        D10                                           m.sup.2 /g    __________________________________________________________________________    A   61.4           28.3              5.3                 0.34                     29.9                        14.5                           4.7                              0.18                                  29.0                                     15.8                                        5.7                                           0.38    B   40.5           16.4              4.5                 0.49                     29.9                        14.5                           4.7                              0.18                                  25.8                                     13.2                                        5.5                                           0.41    C   40.5           16.4              4.5                 0.49                     6.2                        1.0                           0.2                              2.80                                  27.7                                     14.3                                        6.5                                           0.42    __________________________________________________________________________

Within the broad range of reactant particle sizes and surface areas,there is exhibited a strong trend toward equalization of these values inthe synthesized composite oxide feedstocks. It is believed that theisomorphism between the starting AMT powders and the synthesizedcomposite oxide powders is accountable for this equalization. Thesynthesized composite oxides were subjected to XRD phase analysis andEDS elemental analysis. The measured WO₃ /CuWO₄ phase ratio and the W/Cuatomic ratio in all three feedstocks were equivalent to thecorresponding ratios established for the CuWO₄ +0.958 WO₃ composite inTables 2 and 3. Thus, the phase distribution and homogeneity of W--Cucomposite oxides is consistent within large ranges of reactant particlesizes. W--Cu composite oxides having median particle sizes from about 5μm to about 25 μm may be produced from mechanical blends of ammoniumtungstates having median particle sizes from about 5 μm to about 100 μmand oxides/hydroxides of copper having median particle sizes of about0.5 μm to about 20 μm.

II. HYDROGEN REDUCTION OF COMPOSITE OXIDES

Co-reduced W--Cu composite powders made from the above W--Cu compositeoxide mixtures, CuWO₄ +nWO₃ (n>0), exhibit the gray color which ischaracteristic of the color of freshly reduced tungsten powder. Noindication of the presence of copper is observed. This is consistentwith the formation of W--Cu pseudoalloy particles wherein the copperphase has been substantially encapsulated by the tungsten phase.

The reduction of CuWO₄ begins with the reduction of copper followed bystepwise reduction of tungsten:

    CuWO.sub.4 →xCu+Cu.sub.1-x WO.sub.4-x →Cu+(WO.sub.3 →WO.sub.2.9 →WO.sub.2.72 →WO.sub.2 →W)

The reduction of copper from CuWO₄ has a very high thermodynamicprobability and can be completed in the temperature range of 305° C. to488° C.

Substantially higher temperatures are required for tungsten reductionfrom WO₃. For example, it is standard practice to reduce WO₃ in thetemperature range of 840° C. to 900° C. In case of CuWO₄, the presenceof copper lowers the WO₃ reduction temperature. For example, in thepresence of copper, WO₃ may be reduced to tungsten via the stepwisereduction of tungsten suboxides between 661° C. to 750° C.

The discrepancy between temperatures needed to reduce copper from CuWO₄(300° C. to 400° C.) and final stages of tungsten reduction (750° C. to800° C.) is significant and results in segregation of prematurelyreduced copper. Attempts to bring these temperatures closer by loweringthe tungsten reduction temperatures may trigger another undesirableeffect. Stable, nonpyrophoric α-W is formed only by the stepwisesuboxide reduction sequence at temperatures well above 500° C. to 550°C. Below these temperatures, a pyrophoric, unstable β-W, or a W₃ Ophase, is formed as a result of skipping the suboxide sequence and goingthrough a WO₃ →W₃ O→W transition. Thus, the catalytic effect caused bythe presence of copper may promote the formation of pyrophoric β-W.

In order to circumvent copper segregation and the formation ofpyrophoric β-W while achieving a homogeneous distribution of Cu and Wphases in the co-reduced powder, it has been suggested to reduce CuWO₄at once at rather high temperatures (about 700° C.). However, this wouldbe difficult to implement under industrial hydrogen reduction conditionswhere boats containing the composite oxide material have to pass througha temperature transition zone before reaching the final isothermal zonewhere the reduction would occur.

It has been found that the WO₃ -coated CuWO₄ particle phase distributionin the W--Cu composite oxides dramatically influences the reductionkinetics of copper from the composite oxide compared to its reductionfrom CuWO₄. Instead of the 300° C. to 400° C. temperature range, theappearance of copper is shifted to a much more favorable range of 550°C. to 700° C. This leaves less time for copper segregation prior toappearance of tungsten. Additionally, the catalytic effect of thepresence of copper metal phase lowers the WO₃ reduction temperature tobetween 700° C. to 850° C. The copper particles formed during thereduction of the composite oxide serve as sites for deposition oftungsten thereby controlling the phase distribution and size of W--Cucomposite powder. The encapsulation of the copper particles by tungstenprevents further segregation of the copper.

In the tests on hydrogen reduction of CuWO₄ +nWO₃ (n>0) compositeoxides, Inconel boats with composite oxide boatloads of 80 to 160 gramsand bed depths of 3/8" to 3/4" were used. Hydrogen flow rates may befrom about 20 cm/sec to about 300 cm/sec. The tests were carried out ina laboratory tube furnace having thermal zones for gradual temperatureincrease, isothermal hold, and cooling down to ambient temperature. Thereduction kinetics of the composite oxides was studied throughout thestages of temperature increase and isothermal hold of the reductioncycle. The rate of temperature increase from ambient temperature to theisothermal hold temperature varied within a broad range (5° C. to 20°C./minute) characteristic of rates used in industrial reductionfurnaces. Reduction parameters--hydrogen flow rate and velocity, rate oftemperature increase from ambient to isothermal hold, reductiontemperature (isothermal hold), length of reduction--could be controlledfor close simulation of the industrial reduction conditions. Table 5illustrates the hydrogen reduction kinetics of the CuWO₄ +0.958 WO₃composite oxide (Feedstock A, Table 4). The hydrogen reduction wasperformed using bed depths of 3/8" and dry hydrogen with a dew point of-60° C. XRD, Optical Microscopy (OM) and Back-scattered Electron Imaging(BEI) were used to analyze the material at different stages in thereduction process. No phase transition could be detected by XRD, OM orBEI until the composite oxide was exposed to an average temperature of542° C. across the boat length. An increase in surface area of thematerial and appearance of the CuWO₃.19 phase indicated the onset ofcopper reduction which was confirmed by OM. BEI photomicrographs showedthe deposition of copper along the grain boundaries of oxides.

                                      TABLE 5    __________________________________________________________________________    HYDROGEN REDUCTION KINETICS OF THE CuWO.sub.4 + 0.958 WO.sub.3 (15% Cu)              Product Characteristics    Boat Temperature,              Phases Detected                           Surface Area,                                  Degree of    ° C.**              by XRD       m.sup.2 /g                                  Reduction, %    __________________________________________________________________________    25        CuWO.sub.4, WO.sub.3                           0.38   0    542, Average              CuWO.sub.4, CuWO.sub.3.19, WO.sub.3, Cu                           0.93   9.5    682, Average              WO.sub.2.72, WO.sub.2, Cu                           1.48   28.6    768, Average              WO.sub.2, Cu, W                           1.54   47.6    800, After 21              WO.sub.2, Cu, W                           1.31   64.3    Minutes    800, After 34              WO.sub.2, Cu, W                           0.92   85.7    Minutes    800, After 48              WO.sub.2, Cu, W                           0.75   97.0    Minutes    25        Cu, W        0.69   99.5    __________________________________________________________________________     **Average Rate of Temperature Increase From 25° C. to 800°     C. = 13° C./Minute; Residence Time at 800° C. = 60 Minutes

At an average temperature of 682° C., the reduction of copper is closeto completion. A honeycomb of copper deposited along the tungsten oxidegrain boundaries was observed by OM. BEI clearly shows a submicronnetwork of interconnected copper veins and a change in particlemorphology from rounded to dendritic caused by the copper phase changeand partial loss of oxygen. A WO₂.72 phase is observed by XRD showingthat the reduction of WO₃ is proceeding via the WO₃ →WO₂.9 →WO₂.72 →WO₂→W sequence.

At an average temperature of 768° C., the material's surface areareaches its peak value probably because of the completion of the WO₃→WO₂ transition. XRD begins to detect the presence of tungsten. OM andBEI analysis find signs of both copper coalescence and tungstendeposition on copper.

The dominating processes throughout the 800° C. isothermal hold in thereduction cycle are: (1) a decrease in the material's surface arearelated to an increase in the degree of reduction; (2) the coalescenceof copper; (3) WO₂ reduction and deposition of tungsten on coppersurfaces leading to encapsulation of copper particles; and, (4) controlof the size and morphology of the W--Cu composite particles by the sizeand morphology of the copper particles.

The resultant W--Cu composite powder consists of irregularly shapedparticles having a phase distribution wherein the tungsten phasesubstantially encapulates the copper phase. Based on BEI and SNMSanalyses, the thickness of tungsten coating varies with the size andshape of the composite particles and is estimated to range between 0.1and 0.2 μm.

Copper plays an important role in controlling the size of the coppercore and the overall size of the W--Cu composite particle. It was foundthat the particle size of as-reduced W--Cu composite powder increaseswith copper content and ranges from about 0.5 μm to about 2.0 μm (FSSS)as the copper content increases from 2 to 25 wt. %.

Table 6 shows the effect that reduction conditions have on the particlesize of the W--Cu composite powders. CuWO₄ +0.958 WO₃ feedstocks A, B,and C (15% Cu) and a CuWO₄ +0.035 WO₃ feedstock D (25% Cu) were reducedunder identical conditions (800° C. for 1 h. and 28 cm/sec H₂). The FSSSparticle size ranged from 1.52 to 1.75 μm for the unmilled W--Cucomposite powders having 15 wt. % Cu (W-15% Cu). The W--Cu compositepowder having 25 wt. % Cu (W-25% Cu) had a slightly higher FSSS particlesize of 1.8 μm. BEI analysis of the W-25% Cu powder shows larger, lessinterconnected particles compared to W-15% Cu powder.

Feedstock A was reduced at hydrogen flow velocities of 28 and 80cm/second. A 2.86 times increase in hydrogen flow velocity from 28 to 80cm/sec (800° C., 1 h.) caused a decrease in the FSSS particle size ofthe resultant W--Cu composite powder made from feedstock A from 1.75 μmto 0.96 μm. This is similar to the effect observed in the manufacture oftungsten powders.

Feedstocks A, B, C were reduced at two different temperatures, 800° C.and 750° C. The FSSS particle size of the reduced unmilled powder wasdecreased about 1.1 to about 1.4 times by a 50° C. decrease in reductiontemperature. As a consequence of size reduction, the powder surfaceareas and, correspondingly, the oxygen content are increased. Thereduced powders in Table 6 all had oxygen contents less than about 5000ppm. For W--Cu composite powders reduced above about 850° C., excessivecopper coagulation was observed. For powders reduced below about 700°C., the powders exhibited pyrophoricity because they were underreduced,had high surface areas, and had oxygen contents above 5000 ppm. Thus, itis preferable to reduce the W--Cu composite oxides between about 700° C.to about 850° C.

                                      TABLE 6    __________________________________________________________________________    HYDROGEN REDUCTION OF W-Cu OXIDE COMPOSITES    Reduction Parameters                        Unmilled    W-Cu     Hydrogen Flow                    Holding                        Particle Size Distribution, μm                                                Surface    composite         Temp.,             Velocity,                    Time,                        Unmilled    Rod Milled  Area,                                                    O.sub.2,    oxide         ° C.             cm/sec hours                        D90                           D50                              D10                                 FSSS                                    D90                                       D50                                          D10                                             FSSS                                                m.sup.2 /g                                                    ppm    __________________________________________________________________________    A    750 28     1.5 24.9                           8.0                              1.6                                 1.42                                    6.1                                       1.5                                          0.5                                             0.72                                                1.42                                                    1400    B    750 28     1.5 28.8                           9.2                              1.9                                 1.27                                    6.5                                       1.5                                          0.5                                             0.75                                                1.30                                                    1600    C    750 28     1.5 14.1                           5.5                              1.4                                 1.27                                    2.6                                       1.0                                          0.4                                             0.78                                                1.27                                                    1600    A    800 28     1   28.80                           10.60                              3.00                                 1.75                                    5.40                                       2.10                                          0.6                                             1.15                                                0.69                                                    820    B    800 28     1   28.47                           10.41                              3.09                                 1.70                                    7.08                                       1.97                                          0.6                                             1.02                                                0.71                                                    790    C    800 28     1   15.60                           6.40                              2.40                                 1.52                                    3.90                                       1.80                                          0.6                                             1.18                                                0.72                                                    800    A    800 80     1.0 29.0                           10.2                              2.8                                 0.96                                    -- -- -- -- 0.89                                                    1300    A    750 80     1.5 27.1                           7.1                              1.5                                 0.85                                    -- -- -- -- 1.68                                                    1800    A    700 80     2.0 25.8                           9.1                              3.0                                 0.74                                    -- -- -- -- 3.65                                                    3200    D    800 28     1   31.5                           12.6                              2.9                                 1.80                                    7.5                                       2.6                                          0.8                                             1.25                                                0.56                                                    1300    __________________________________________________________________________

Composite metal powders consisting of two or more metal phases are moreprone to oxidation and pyrophoricity than powders of a single metal.Passivation of the reduced W--Cu composite powders with nitrogenimmediately after reduction dramatically decreases the powders tendencyto oxidize and become pyrophoric. As an example, Feedstock A which wasreduced at 700° C. using a hydrogen flow velocity of 80 cm/second had asubmicron FSSS (0.74 μm), a high surface area and an oxygen content of3200 ppm. After having been exposed to air for about 2 hours, thereduced powder became pyrophoric. When a similarly reduced powder wasplaced under nitrogen and passivated for about 2 hours, the reducedpowder did not show any signs of pyrophoricity after 24 hours ofexposure to air.

III. CONSOLIDATION OF W--Cu COMPOSITE POWDERS

The high surface area of the W--Cu composite powders increases theoxygen content in the powder compact. Surface oxides on thetungsten-copper interface present a significant problem in manufacturinga dense pseudoalloy with a continuous W--Cu structure. It has beenestablished that at 1100-1200° C. oxygen-saturated copper does not wettungsten. Oxygen-free copper also does not wet tungsten having surfaceoxides in the form of WO₃ or WO₂ (OH)₂ ! on tungsten-copper interface.Cleaning oxygen from W--Cu powder compacts presents a fundamentalproblem in manufacturing the pseudoalloys. This problem is complicatedby the "hydrogen disease" of copper and a high tungsten affinity foroxygen.

The data in Table 7 illustrate the problem of "hydrogen disease" incopper.

                  TABLE 7    ______________________________________    Temperature, ° C.                1065          1100    ______________________________________    Oxy- Form of    Cu + 3.5% Cu.sub.2 O                                  Cu + 0.54 Cu.sub.2 O or less    gen  Existence  (eutectic)    after hydrogen cleaning         Concentration,                    0.39          0.06 or less         %    Effect      Swelling in hydrogen.                              No swelling in                NO wetting of hydrogen.                tungsten.     Good wetting of                              tungsten.    ______________________________________

Water molecules formed during hydrogen reduction of trapped oxygen causeswelling of copper and W--Cu compacts. The compacts must have 15 to 20%porosity to allow free escape of water molecules and good hydrogencleaning of oxygen. However, this does not completely solve the problem.In the presence of water vapor, complex WO₂ (OH)₂ ! molecules form onthe surface of tungsten. Therefore, forcing oxygen out of the W--Cusystem and maintaining a clean interface between the metal phasespresents a complex metallurgical problem.

Critical applications in electrical/electronic engineering require W--Cupseudoalloys with superior structural homogeneity and performance underincreased thermal and mechanical stresses. For metals, one of the mostfundamental physical properties is the electrical conductivity. It canprovide information on chemical composition, structural uniformity,electrical and mechanical properties, and material response totemperature changes. Of particular interest for W--Cu pseudoalloys isthe excellent correlation between their electrical and thermalconductivity. The latter property is of critical importance indeveloping thermal management materials for electronics and is difficultto measure.

Modern methods of measuring electrical conductivity are based onnondestructive eddy current (EC) measurement principles. The electricalconductivities of the W--Cu pseudoalloys produced from the W--Cucomposite powders were measured using an EC conductivity meter TypeSigmatest D 2.068 manufactured by the Institut Dr. Forster in Germany.Electrical conductivity measurements were made according to theInternational Accepted Conductivity Standard (IACS) issued byInternational Electronical Commission in 1914 for high conductivitycopper as 100% IACS.

Sintering activity is the basic property which controls powderconsolidation. To eliminate complicating factors, sintering activityshould be tested using deagglomerated powders having finite particledimensions. From Table 6, it can be seen that the average medianparticle size of deagglomerated (rod-milled) W-15% Cu composite powdersreduced at 800° C. from feed-stocks A, B and C is about 2 μm. Theserod-milled powders were sintered to produce W--Cu pseudoalloy articles.Consolidation may be carried out in hydrogen, dissociated ammonia orvacuum.

About 0.5 wt. % of an organic lubricant (Acrawax) was blended with theW--Cu composite powder to impart pressability. Acrawax C is the tradename for ethylene bis-stearamide manufactured by Lonza Co.,Fair Lawn,N.J. This white organic lubricant powder decomposes between 300 to 400°C. without leaving any harmful residue in the compact. Round samples(D×H=14.8 mm×3 to 4 mm) were die-pressed to a green density of betweenabout 40 to about 75% of theoretical density (TD) by applying acompacting pressure in the range of 18.8 to 206.8 ksi. Powder compactswere sintered under flowing dry hydrogen in a molybdenum tube inside ahigh temperature laboratory furnace having automated control of theheating cycle. The sintering cycle consisted of a combination oftemperature increases and isothermal holds. A temperature rate increaseof 10° C./min was used between 120 minute isothermal holds at 850, 950,1050, 1100, 1150, 1200, and 1250° C.

No appreciable linear shrinkage of the compacts was observed through950° C. Above this temperature and through the final sinteringtemperature, the compacts experienced varying rates of shrinkage. Table8 gives the shrinkage of the compacts as a function of sinteringtemperature and compacting pressure.

                  TABLE 8    ______________________________________    Shrinkage of Compacts from W-15% Cu Composite Powder (D50 = 2 μm)                               Ratios of Incremental    Compacting            Linear shrinkage,  to Total Shrinkage    Pressure,            %, vs. Sintering Temperature, ° C.                               (×100, %)    ksi     1050   1100   1150 1200 1250 1050/1250                                                1100/1250    ______________________________________    18.8    5.00   18.35  21.01                               22.05                                    22.23                                         22.49  82.55    37.6    5.23   16.85  18.88                               19.73                                    19.82                                         26.39  85.02    56.4    4.68   15.23  17.49                               17.73                                    17.92                                         26.12  84.99    75.2    4.48   14.09  16.11                               16.12                                    16.36                                         27.38  86.12    94.0    4.19   12.93  14.69                               15.03                                    15.23                                         27.51  84.90    112.8   3.94   12.52  13.66                               13.96                                    14.12                                         27.90  88.67    131.6   3.50   11.37  12.67                               12.81                                    13.15                                         26.62  86.46    150.4   3.64   10.43  11.68                               11.82                                    12.08                                         30.13  86.34    169.2   3.48   9.88   10.84                               10.90                                    11.34                                         30.69  87.13    188.0   3.20   8.76   10.02                               10.30                                    10.57                                         30.27  82.88    206.8   3.04   8.31   9.48 9.71 9.91 30.68  83.85    ______________________________________

In each sintering run, the total shrinkage progressively decreased withincreasing compacting pressure and green density. However, theincremental shrinkage of compacts exhibited an opposite trend: ratios ofincremental shrinkage to total shrinkage at 1250° C. were progressivelyhigher with the increase in compacting pressure and better W--Wcontacts. The incremental shrinkage of samples exhibited values of up to88.67% in the temperature interval of 950-1100° C. Appearance of liquidcopper at 1100° C. could not be considered as the only factorresponsible for such high rates of shrinkage since this temperature isquite low for sintering W--Cu systems. Therefore, such high sinteringrates could only be attributed to rapid local densification of submicrontungsten particles at the stage of solid-state sintering. Suchdensification rates should adversely affect the sintering process andresult in an unevenly sintered tungsten framework. In turn, this shouldcause uneven densification and copper bleedout at the liquid-phasesintering stage. Indeed, this has been observed at a sinteringtemperature of 1250° C.

Referring to the pressing-sintering curves in FIG. 7, a growth ofelectrical conductivity can be observed with the increase inliquid-phase sintering temperature and shrinkage of the samples.However, despite the continued shrinkage at 1250° C., there is a drop inelectrical conductivity of samples sintered at this temperature. It isaccompanied by appearance of copper areas at the sample surfaces. Sincesintered densities of 98-98.5% TD have been reached at this temperature,the bleedout of copper could only be attributed to nonuniformdensification, in particular, to high rates of shrinkage andstructurally inhomogeneous tungsten framework formed at the stage ofsolid-state sintering.

The complete elimination of copper bleedout at high sintered densitiesis achieved by reducing the rate of temperature increase during thesolid-state sintering stage. A preferred sintering method for thetungsten-coated copper composite powders includes the steps of: (1)lubricant or binder removal from the consolidated powder at 300-500° C.;(2) residual oxygen removal at 800-950° C.; (3) in situ sintering of atungsten framework at very low rates of temperature increase (about 1°C./minute to about 5° C./minute)in the range of 950-1080° C.; (4)residual oxygen removal from molten copper at 1080-1130° C.; and (5)internal infiltration of the tungsten framework and densification of thepseudoalloy at 1150-1600° C.

The phase distribution in the composite powder particles (a submicrontungsten coating over a micron-size copper core) predetermines the veryhigh densification activity at the liquid-phase sintering stage.Numerous sintering experiments verified that the densificationtemperature should be a function of the powder copper content asfollows:

                  TABLE 9    ______________________________________    Copper Content, Temperature,    wt. %           °0 C.    ______________________________________    20-25           1150-1200    15-20           1200-1250    10-15           1250-1300     5-10           1300-1350    2-5             1350-1600    ______________________________________

Tungsten-copper composite oxides synthesized from spray-dried AMT andCu₂ O and having a relative copper content in the range of 5 to 25 wt. %Cu were reduced in a laboratory hydrogen reduction furnace at 800° C.using a hydrogen flow velocity of 28 cm/sec. The co-reduced W--Cucomposite powders had the following typical characteristics:

D50 Unmilled--6.4-12.6 μm

D50 Rod-Milled--1.8-2.6 μm

BET--0.55-0.75 m² /g

O₂ --800-1300 ppm

The as-reduced powders without prior deagglomeration were pressed intoround samples as above. The sintering cycle consisted of one hourduration isothermal holds at 450, 850, 950, 1100° C. and one of thealloy densification temperatures based on its copper content (Table 10):

                  TABLE 10    ______________________________________    Copper Content, Temperature,    wt. %           ° C.    ______________________________________    25              1200    20              1250    15              1300    10              1350     5              1400    ______________________________________

The rate of temperature increase between isothermal holds was 4° C./min.Between 950 to 1080° C., the rate of temperature increase was limited to1° C./min. After completion of densification, the parts were cooled downto room temperature at a rate of 4° C./min.

The tests with systematic control of copper content and sinteringtemperature confirmed that coreduced W--Cu composite powders possess ahigh sintering activity even without prior deagglomeration. No copperbleedout was observed. High sintered densities were achieved for W--Cupseudoalloys with 10 to 25 wt. % of copper. The amount of liquid phasewas, apparently, inadequate for achieving a high sintered density in theW-5 wt. % Cu alloy.

FIG. 8 shows the electrical conductivities for these samples as afunction of compacting pressure. Due to insufficient copper content, theelectrical conductivity in the W-5 wt. % Cu pseudoalloy keeps increasingwith the compacting pressure within the whole range of tested pressures.For the W-10 wt. % Cu samples, the stabilization of electricalconductivity coincides with the stabilization of sintered density andoccurs at a compacting pressure of about 110-115 ksi. Progressivelylower compacting pressures are needed for stabilization of electricalconductivity and sintered density for pseudoalloys with higher coppercontent. They are, correspondingly, 75-80 ksi for W-15 wt. % Cu, 55-60ksi for W-20 wt. % Cu, and 40-50 ksi for W-25 wt. % Cu.

Table 11 summarizes the analyses of the microstructural characteristicsof the W--Cu pseudoalloy samples. BEI photomicrographs showing themicrostructural cross-sections of the W--Cu pseudoalloys show uniformlydistributed and densely packed tungsten areas having sizes less thanabout 5 μm and copper areas having sizes less than about 10 μm.

Somewhat large sintered tungsten areas and isolated copper areas arecharacteristic of the W-5 wt. % Cu system. The phase distributionuniformity and interconnection greatly improves upon increasing thecopper content to 10 wt. % and, especially, to 15 wt. %. However, highercopper contents result in developing scattered copper areas. Still, thetungsten and copper phases remain fully interconnected. The size oftungsten areas in the photomicrographs progressively decreases with theamount of copper from below 5 μm at 5 wt. % Cu to below 1 μm at 25 wt. %Cu.

A similar analysis of a W--Cu pseudoalloy formed by infiltrating asintered tungsten skeleton with copper showed a much coarsermicrostructure. Analysis of the microstructural cross-section showedthat the average size of the tungsten areas was 10 to 15 μm and the sizeof the copper areas was 15 to 25 μm. Thus, the W--Cu pseudoalloys formedby pressing and sintering the tungsten-coated copper composite powdershave much finer microstructures than infiltrated W--Cu pseudoalloys.Furthermore, W--Cu pseudoalloys having a copper content less than 10 wt.% cannot be successfully made by infiltration techniques.

                                      TABLE 11    __________________________________________________________________________    Microstructural Characteristics of the W-Cu Pseudoalloys    Copper         Tungsten Network                             Copper Network    Content,       Size of                 Size of    wt. %         Structural Uniformity                   areas, μm                        Shape                             State         areas, μm    __________________________________________________________________________     5   Large sintered W                   ≦5                        Angular                             Isolated within W network                                           5-10         areas; isolated Cu  boundaries         areas    10   Some sintered W                   ≦3                        Angular                             Some interconnected/mostly                                           ≦5         areas; uniform      isolated within W network         distribution of Cu  boundaries    15   Uniform distribution                   ≦2                        Mostly                             Interconnected with some                                           ≦2         of phases      Rounded                             clusters of W areas    20   Mostly uniform;                   ≦2                        Rounded                             Highly interconnected with few                                           ≦5         several large Cu    multiple W areas         areas    25   Uniform; scattered Cu                   ≦1                        Rounded                             Totally interconnected                                           ≦5         areas of various         sizes     15**         Highly uniform                   ≦1                        Rounded;                             Highly interconnected with no                                           ≦2         distribution of                        very isolated W areas         phases         uniform    __________________________________________________________________________     **W-Cu composite powder was milled and spray dried prior to sintering

Table 12 compares the electrical conductivities of W--Cu pseudoalloysmade by sintering the co-reduced W--Cu composite powders withpseudoalloys made by pressing, sintering, and infiltrating a tungstenskeleton with copper. Appreciably higher electrical conductivities, andtherefore higher thermal conductivities, are achieved in W--Cupseudoalloys made from the W--Cu composite powders having W-coatedcopper particles.

                  TABLE 12    ______________________________________                     Electrical Conductivity, % IACS                     of Pseudoalloy Made:    Copper Content In      By Cu    Pseudoalloy, %         Infiltration    By    By       From Composite                               of a Tungsten                                         Conductivity    Weight          Volume   Powder      Skeleton  Ratio    ______________________________________    5     10.2     27.6        --        --    10    19.3     36.9        35.0      1.054    15    27.5     41.2        --        --    20    35.0     44.2        41.0      1.078    25    42.0     48.9        45.0      1.086    ______________________________________

Spray-dried, lubricated, flowable powders which can be formed intovarious complex shapes are considered an ideal powder source formanufacturing technologies involving pressing, molding, rolling, andextrusion. As a rule, powders are subjected to deagglomeration (milling)prior to spray drying which significantly improves their uniformity.

Samples of a CuWO₄ +0.958 WO₃ composite oxide (15% Cu) synthesized fromspray-dried AMT and Cu₂ O were reduced in a D-muffle furnace at 800° C.(Type 1) and in a tube furnace at 750° C. (Type 2) under industrialconditions. The co-reduced W--Cu composite powders were milled for 30minutes in a water slurry using Type 440C stainless steel balls asmilling media. Carbowax-8000, a high molecular weight polyethyleneglycol manufactured by Union Carbide Corporation, was used as a binder.The Carbowax-8000 was added to the slurry to give a 2.5 wt. % binderconcentration in the spray-dried powder. Spray drying yielded aspherical powder having excellent flowability and pressability and a 90%-60 +200 mesh fraction. Rectangular (16×18×5 mm) samples weighing 20 geach were pressed at 70 ksi from both types of spray-dried powder. Thecompacts were subjected to the preferred sintering method describedpreviously. Table 13 gives the average test results for pseudoalloysmade from Types 1 and 2. No adverse effects on sintering properties wereobserved at the carbon concentrations remaining in the compacts afterdewaxing. Good sintered density, electrical conductivity, and completeelimination of any signs of copper bleedout were achieved in thesintered samples. A representative sample microstructure ischaracterized in Table 11 and shown in FIG. 9.

FIG. 9 is a BEI photomicrograph of a W--Cu pseudoalloy made from amilled and spray dried tungsten-coated copper composite powder having 15wt. % Cu which was sintered at 1250° C. to 98.2% of theoretical density.The tungsten areas in the microstructural cross-section appear white andthe copper areas appear dark. The photomicrograph clearly shows theuniform distribution of the tungsten and copper areas as well as thehighly interconnected structure of the pseudoalloy.

                                      TABLE 13    __________________________________________________________________________                       Compact Properties    Powder Properties  Carbon Content                               Sintered                                   Electrical    Type       FSSS,           BET,              O.sub.2,                 Sintering                       After Dewaxing,                               Density,                                   Conductivity,    No.       μm           m.sup.2 /g              ppm                 Temp., ° C.                       ppm     % TD                                   % IACS    __________________________________________________________________________    1  1.22           0.80              1100                 1300  190     98.66                                   40.00    2  1.27           1.91              3160                 1250  333     98.17                                   40.86    __________________________________________________________________________

IV. CERAMIC METALLIZATION

Ceramic metallization is one of the most critical and precisionoperations in electronics. It consists of forming a metal layer onceramic surfaces with a ceramic-to-metal joint which is both strong andvacuum-tight. Metallized ceramic surfaces are used for ceramic-to-metaland ceramic-to-ceramic connections and for wiring conductor layers(paths) in electronic technology. Metallized ceramic is produced byapplying a metallizing paste to sintered or unsintered ceramicsubstrates (mostly alumina or beryllia) and firing the paste to yield anadherent metallized area. Co-firing a paste with the "green"(unsintered) ceramic is economically advantageous since it eliminatespresintering the ceramic and results in better quality metallization. Anumber of paste formulations and metallization techniques have beendisclosed in U.S. Pat. Nos. 3,620,799, 4,493,789 and 4,799,958.

Generally, a paste for ceramic metallization consists of three basiccomponents: metal powder, frit material, and a binder. Forlow-temperature metallization (up to 1200-1250° C.) of presinteredceramics metals like silver, palladium, nickel and copper may be used.High-temperature metallization or co-firing with an unsintered ceramic(up to 1600-1700° C.) requires the use of refractory metals (W, Mo)alone, or their mixtures with Mn or Pt. Frit material comprises oxidesand silicates. Due to their glassy nature, frit components are fillingthe pores in the ceramic substrate and metal structure and help making avacuum tight ceramic-to-metal joint. Metal powders and frit materialsare held together by binders like ethyl cellulose, nitrocellulose, epoxyresins, and many other organic binding agents. Usually, the componentsare milled and mixed for several hours to decrease their particle sizeand make a homogeneous blend. Finally, the consistency of themetallizing paste is adjusted by a vehicle which is actually a solventcompatible with the binder. For instance, for ethyl cellulose binders,ethylene glycol dibutyl ether is preferred. Consistency adjustment isrequired according to the method of application of the paste on ceramicsurfaces. Screen printing technique is usually employed for ceramicwiring boards and packages; other methods like painting, spraying,dipping, etc. are also broadly applied.

Refractory metals require high temperatures for metallization. Theelectroconductivity of the tungsten conductor layer is increased inproportion to the sintering temperature. Higher electrical conductivitymakes it possible to sinter finer wires (paths or lines) and increasethe packaging density of the wiring boards. However, metallizingtemperatures above 1750° C. are very detrimental to the ceramic. Even attemperatures between 1450 and 1700° C. the ceramic exhibits grain growthwhich may be detrimental to its strength, and hot creep to the pointwhere creepage is a severe problem.

All three objectives--(1) lowering the co-firing temperature formetallizing an unsintered ceramic; (2) increasing the electricalconductivity of the metallized wires; and (3) improving the packagingdensity of wiring boards--can be accomplished by using thetungsten-coated copper composite powder in the metallizing pasteformulations.

The W-5 wt. % Cu powder can be sintered at temperatures as low as 1400°C. to high densities and electrical conductivities. However, coppercontent in the range of 2 to 15 wt. % can be used for broadening therange of metallizing temperatures to 1200-1600° C. By controlling thecopper content in the W--Cu composite powder used in pastes, theelectrical conductivity of the metallized conductive lines can becontrolled. Moreover, due to excellent structural homogeneity of W--Cupseudoalloys from the W-coated copper composite powders, the width ofthe metallized conductive line can be decreased thereby improving thepackaging density of a semiconductor assembly. Indeed, 0.002 inch linewidths have been successfully metallized using the W-15 wt. % Cu powder.

While there has been shown and described what are at the presentconsidered the preferred embodiments of the invention, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

We claim:
 1. A method for forming a homogeneous W--Cu pseudoalloycomprising pressing a tungsten-coated copper composite powder to form acompact and sintering the compact, the tungsten-coated copper compositepowder comprising individual particles having a tungsten phase and acopper phase wherein the tungsten phase substantially encapsulates thecopper phase.
 2. The method of claim 1 wherein sintering the compactcomprises forming a sintered tungsten framework in a solid-statesintering stage followed by internal infiltration of the sinteredtungsten framework by liquid copper during a liquid-phase sinteringstage.
 3. The method of claim 1 wherein the compact is sintered withoutcopper bleedout.
 4. The method of claim 3 wherein the sinteringcomprises subjecting the compact to a temperature cycle, the temperaturecycle comprising (1) increasing temperature from ambient temperature toa temperature sufficient to cause solid-state sintering, (2) slowlyincreasing the temperature at a rate of about 1° C./minute to about 5°C./minute until liquid-phase sintering begins, and (3) increasing thetemperature to a temperature sufficient to complete densification of thecompact.
 5. The method of claim 4 wherein the temperature sufficient tocomplete densification is from 1150° C. to 1600° C.
 6. The method ofclaim 5 wherein the temperature sufficient to complete densification isdetermined by the amount of copper in the tungsten-coated coppercomposite powder.
 7. The method of claim 1 wherein the tungsten-coatedcomposite W--Cu powder is mixed with a binder prior to pressing.
 8. Themethod of claim 1 wherein sintering the compact comprises (1) removing abinder from the compact at 300-500° C.; (2) oxygen removal from thecompact at 800-950° C.; (3) solid-state sintering of a tungstenframework at a very low rate of temperature increase in the range of950-1080° C.; (4) oxygen removal from molten copper at 1080-1130° C.;and (5) internal infiltration of the tungsten framework anddensification of the pseudoalloy at 1150-1600° C.
 9. The method of claim8 wherein the rate of temperature increase during the solid-statesintering is from about 1° C./minute to about 5° C./minute.